cant get the sub-surf right (Blender guru beginner modeling tutorial)
I guess your mesh has something different in the bottom: I think you have a single bottom face (N-gon), which is causing this issue...

if you have this (sorry for the horrible mesh, I'm before breakfast...)



subsurf behaves well



while if you have a single N-gon face



subsurf behaves like in your model



if unsure, share your model through 

http://blend-exchange.giantcowfilms.com/

then edit your question and post the provided link...

edit:
after getting your file I found that this model has (still) several issues: at the base there are still n-gons, ruining the subsurf effect, but overall it has some other modeling issues... giving aso bad mirroring results.

About the ngons, this what you have one side



and the opposite is even worse



then below you have some faces with inverted normals



then again, its quite irregular on lines that are supposed to be quite linear instead: this ruins both mirror and subsurf

eg: here









Nothing terrible, and it can be corrected, but I suggest you to try to model keeping an eye on such things, because it helps you model better, and get better results. First, I recalculated normals (select  all in edit mode, then CTRLN)

Then I tried to remove those ngons, getting something like this



if you enable subsurf now, it looks better, but not at its best: 



it lacks some subdivision at the base, eg:



with this further subdivision, 



the base becomes more defined when you subsurf later



Here is the file a bit corrected (imho, it could still be improved)



Hth, Marco